The presently disclosed embodiments are directed to an imaging member used in electrostatography. More particularly, the embodiments pertain to an electrostatographic imaging member with an improved anticurl backing layer comprising a low surface energy polymeric material and a process for making and using the member.
In electrostatographic reproducing apparatuses, including digital, image on image, and contact electrostatic printing apparatuses, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles and pigment particles, or toner. The electrophotographic imaging members may include photosensitive members (photoreceptors) which are commonly utilized in electrophotographic (xerographic) processes, in either a flexible belt or a rigid drum configuration. Other members may include flexible intermediate transfer belts that are seamless or seamed, and usually formed by cutting a rectangular sheet from a web, overlapping opposite ends, and welding the overlapped ends together to form a welded seam. These electrophotographic imaging members comprise a photoconductive layer comprising a single layer or composite layers.
One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer (CTL). Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between a contiguous CTL and the supporting conductive layer. Alternatively, the CTL may be sandwiched between the supporting electrode and a photoconductive layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged in the dark with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a member such as paper.
In the case where the charge-generating layer (CGL) is sandwiched between the CTL and the electrically conducting layer, the outer surface of the CTL is charged negatively and the conductive layer is charged positively. The CGL then should be capable of generating electron hole pair when exposed image wise and inject only the holes through the CTL. In the alternate case when the CTL is sandwiched between the CGL and the conductive layer, the outer surface of Gen layer is charged positively while conductive layer is charged negatively and the holes are injected through from the CGL to the CTL. The CTL should be able to transport the holes with as little trapping of charge as possible. In flexible web like photoreceptor the charge conductive layer may be a thin coating of metal on a thin layer of thermoplastic resin.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, however, degradation of image quality was encountered during extended cycling. The complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers used in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, an optional blocking layer, an optional adhesive layer, a charge generating layer, a CTL and a conductive ground strip layer adjacent to one edge of the imaging layers, and an optional overcoating layer adjacent to another edge of the imaging layers. Such a photoreceptor usually further comprises an anticurl backing layer on the side of the substrate opposite the side carrying the conductive layer, support layer, blocking layer, adhesive layer, charge generating layer, CTL and other layers.
After applying the various coating for the multilayered organic photoconductors, the resulting web tends to spontaneously “curl” once the coating solvents evaporate. Curl is primarily due to dimensional contraction of the applied CTL coating from the point in time when the applied CTL coating solidifies and adheres to the underlying surface. Once this solidification and adhesion point is reached, the evaporation of coating solvent further causes continued shrinking of the applied CTL coating due to volume contraction. Removal of additional solvent will cause the coated web to curl toward the applied CTL, because the substrate (often polyethylene terephthalate) does not undergo any dimensional changes. This shrinking occurs isotropically, e.g., three-dimensionally.
Curling of a photoreceptor web is undesirable because it hinders fabrication of the web into cut sheets and subsequent welding into a belt. An anticurl backing layer having a curl equal to and in the opposite direction to the applied layers is applied to the reverse side of the active photoreceptor to eliminate the overall curl of the coated device by counteracting the curl due to the mismatch of the thermal expansion of the photoreceptor substrate and the CTL. The anticurl backing layer counter compensates the curl so as to promote the photoreceptor to lay flat. However, common anticurl backing layer formulations are not always sufficient in the larger, more complex systems.
Curling is also undesirable because different segments of the imaging surface of the photoconductive member are located at different distances from charging devices, causing non-uniform charging. In addition, developer applicators and the like, during the electrophotographic imaging process, may all adversely affect the quality of the ultimate developed images. For example, non-uniform charging distances can manifest as variations in high background deposits during development of electrostatic latent images near the edges of paper. Coatings may be applied to the side of the supporting substrate opposite the photoconductive layer to counteract the tendency to curl, however, difficulties have been encountered with these anticurl coatings. Anticurl layers will also occasionally delaminate due to poor adhesion to the supporting substrate. Delamination is particularly troublesome in high speed automatic copiers, duplicators and printers which require extended cycling of the photoreceptor belt. For example, delamination has occurred in as few as 8,000 cycles. Moreover, delamination is accelerated when the belts are cycled around small diameter rollers and rods.
Since the anti-curl coating is an outermost exposed layer, it has further been found that during cycling of the photoconductive imaging member in electrophotographic imaging systems, the relatively rapid wearing away of the anti-curl coating also results in the curling of the photoconductive imaging member. In some tests, the anti-curl coating was completely removed in 150 thousand to 200 thousand cycles. This problem is even more pronounced when photoconductive imaging members in the form of webs or belts are supported in part by stationary guide surfaces which causes the anti-curl layer to wear away very rapidly and produce debris which scatters and deposits on critical machine components such as lenses, corona charging devices and the like, thereby adversely affecting machine performance.
In other machines the electrostatic charge builds up due to contact friction between the anti-curl layer and the backer bars. The charge increases the friction and thus requires higher torque to pull the belts. In full color machines with 10 pitches this can be extremely high due to large number of backer bars used. At times, one has to use two drive rollers rather than one which are to be coordinated electronically precisely to keep any possibility of sagging. In other cases, the electrostatic charge build up can be so high as to cause sparking.
Many attempts have been made to overcome the above problems but not without leading to additional problems. For example, although the addition of micro-crystalline silica, at a 10 weight percent level in the anti-curl layer has been found to decrease charge transport layer/anti-curl layer surface contact friction, excessive welding horn wear is observed when this electrophotographic imaging member belt is fabricated by the ultrasonically welding of overlapped ends of an imaging member sheet. It also can cause excessive wear of backer bars.
Another problem encountered in the conventional belt photoreceptors using a bisphenol A polycarbonate anti-curl backing layer that are extensively cycled in precision electrostatographic imaging machines utilizing belt supporting backer bars, is an audible squeaky sound generated due to high contact friction interaction between the anti-curl layer and the backer bars. Further, cumulative deposition of anti-curl layer debris onto the backer bars may give rise to undesirable defect print marks formed on copies which forces the photoreceptor upwardly and interferes with the toner image development process. On other occasions, the anti-curl layer wear causes debris accumulation on the backer bars to gradually increase the dynamic contact friction between these two interacting surfaces, interfering with the duty cycle of the driving motor to a point where the motor eventually stalls and belt cycling prematurely ceases.
A known anticurl backing layer used in the printing apparatuses includes polytetrafluoroethylene (PTFE) dispersed in polymer binder solution. PTFE is commonly incorporated to reduce the friction of the photoreceptor to the backer bars. This formulation, however, is unable to address the friction in larger copiers or printers because of a problem with stability of the coating solution. PTFE forms an unstable dispersion in a polymer solution, commonly a bisphenol A polycarbonate polymer solution, and tends to settle in the mix tanks if not continuously stirred. The dispersion problem can result in an anticurl backing layer with insufficient and variable PTFE along the length of the coated web, and thus, inadequate reduction of friction over the backer bars in the copiers or printers. This causes significant complications in the larger copiers or printers, which often include so many backer bars that the high friction increases the torque needed to drive the belt. Consequently, two driving rollers are included and synchronized to prevent any misregistration. The additional components result in high costs for producing and using these larger printing apparatuses. Thus, if the friction could be reduced, the apparatus design in these larger printing apparatuses could be simplified with less components, resulting in significant cost savings. The inventions discussed above also contemplate dispersion of other particles, such as silica or PTFE in the solution of polymeric binder. However, these generally have a problem of instability of solutions and thus the shelf life, and consequently, needs to be constantly stirred. The pure polymeric binder solution does not have the stability problem but does have other problems, as discussed above.
In U.S. Pat. No. 5,069,993, an exposed layer in an electrophotographic imaging member is provided with increase resistance to stress cracking and reduced coefficient of surface friction, without adverse effects on optical clarity and electrical performance. The layer contains a polymethylsiloxane copolymer and an inactive film forming resin binder. Various specific film forming resins for the anti-curl layer and adhesion promoters are disclosed.
U.S. Pat. No. 5,021,309 shows an electrophotographic imaging device, with material for an exposed anti-curl layer has organic fillers dispersed therein. The fillers provide coefficient of surface contact friction reduction, increased wear resistance, and improved adhesion of the anti-curl layer, without adversely affecting the optical and mechanical properties of the imaging member.
U.S. Pat. No. 5,919,590 shows An electrostatographic imaging member comprising a supporting substrate having an electrically conductive layer, at least one imaging layer, an anti-curl layer, an optional ground strip layer and an optional overcoating layer, the anti-curl layer including a film forming polycarbonate binder, an optional adhesion promoter, and optional dispersed particles selected from the group consisting of inorganic particles, organic particles, and mixtures thereof.
In U.S. Pat. No. 4,654,284 an electrophotographic imaging member is disclosed comprising a flexible support substrate layer having an anti-curl layer, the anti-curl layer comprising a film forming binder, crystalline particles dispersed in the film forming binder and a reaction product of a bifunctional chemical coupling agent with both the binder and the crystalline particles. The use of VITEL PE 100 in the anti-curl layer is described.
In U.S. Pat. No. 6,528,226 a process for preparing an imaging member is disclosed that includes applying an organic layer to an imaging member substrate, treating the organic layer and/or a backside of the substrate with a corona discharge effluent, and applying an overcoating layer to the organic layer and/or an anticurl backing layer to the backside of the substrate.
Thus, electrostatographic imaging members comprising a supporting substrate, having a conductive surface on one side, coated over with at least one photoconductive layer and coated on the other side of the supporting substrate with an anti-curl layer, may exhibit deficiencies which are undesirable in advanced automatic, cyclic electrostatographic copiers, duplicators, and printers. While the above mentioned electrostatographic imaging members may be suitable for their intended purposes, further improvement on these electroreceptors are desirable. For example, there continues to be the need for improvements in such systems, particularly for an imaging member that includes an improved anticurl backing layer that sufficiently counters curling and reduces friction, even in larger printing apparatuses.